ESTRO 2025 - Abstract Book

S617

Clinical - Breast

ESTRO 2025

remains unclear. We aim to evaluate the impact of Stereotactic Body Radiation Therapy (SBRT) on delaying the switch to a new systemic therapeutic line in patients with oligometastatic breast cancer (OMBC).

Material/Methods: We conducted a retrospective single-institution study including patients with OMBC (synchronous or metachronous), between 2018 and 2021, with a number of metastases ranging from 1 to 5 at the time of SBRT. We excluded patients with > 5 active simultaneous metastases and those with a concomitant change in systemic treatment . Active intracranial metastases were accepted. The primary endpoint was the time to the next systemic treatment (TTST). Secondary endpoints were local control (LC), progression-free survival (PFS), overall survival (OS), and toxicities according to the CTCAE v5.0. Results: Over four years, we treated a total of 108 metastases in 76 patients. The median follow-up was 42.8 months (IQR: 32.3-53.6 months). Patient and lesion characteristics are summarized in Table 1 . Among all patients, 56 received SBRT on one site, 12 on two sites, eight on three sites, and one on four sites. Median BED 10Gy was 51.3 Gy (range 22.5-85.5Gy). Fifty-five patients (72.3%) had a change of systemic treatment during the follow-up period. Median TTST was 13.7 months (95% CI: 8.2-22.5 months). Overall, 21 lesions were not controlled by SBRT (19.4%), corresponding to a local control rate of 90.5% (95% CI: 85.0-96.3%) at 1-year and 84.3% (95% CI: 77.5-91.7%) at 2-years. Fifty-nine patients (77.6%) had a progression, with a median PFS of 9.8 months (95% CI: 4.9-17.4 months). Figure 1 shows the PFS curve . Among them, 38 (64.4%) had distant progression only, 20 (33.9%) had both distant and SBRT site progression, and only one (1.7%) had progression limited to the SBRT site. The 1-year OS was 94.7%, the 2-year OS was 86.8% and the median OS was not reached. There were three cases of radionecrosis in a single patient. Two out of three were treated surgically. No other relevant toxicities were observed.